1. Field of the Invention
The present invention relates to an exposure control apparatus and an exposure control method which measure the subject brightness and perform exposure control in an imaging device or the like.
2. Description of Related Art
An imaging device such as a camera controls the shutter speed value, aperture value, ISO sensitivity value or the like on the basis of the result of measurement of the subject brightness so as to perform exposure near the appropriate exposure. During video shooting, the brightness of the field may vary over time, or the subject itself may vary. There is proposed an imaging device which always reproduces brightness and darkness faithfully in accordance with such variation (see Japanese Patent Laid-Open No. 06-245139 (hereafter referred to as “Patent Literature 1”)).
The imaging device disclosed in Patent Literature 1 analyzes the situation of the variation of the subject on the basis of the variation of the image signal level, and adjusts the amount of exposure by controlling the aperture value on the basis of the analysis result. Adjusting the amount of exposure allows faithful reproduction of brightness and darkness.
The imaging device disclosed in Patent Literature 1 maintains the aperture value or drives the aperture so as to achieve the appropriate exposure, in accordance with the variation of the shooting scene. However, Patent Literature 1 does not take into account the brightness variation that occurs near the appropriate exposure due to exposure control when shooting a video.
Referring to FIG. 4, brightness variation that occurs near the appropriate exposure due to exposure control will be described. The graph (a) illustrated at the upper part of FIG. 4 indicates temporal variation of the brightness of the shooting environment, with the brightness staying at BV0 until a time point t0 and falling to a brightness BV1 between time points t0 and t1 in the example. Under such a situation, when the brightness of the shooting environment starts falling at the time point t0, the amount of exposure of the camera slowly follows the falling from an amount of exposure EV1. Although the amount of exposure falls slowly as described above, there is no exposure control performed to obtain the appropriate amount of exposure EV0.
It is originally ideal to perform exposure control to the appropriate exposure level. However, the conventional method of varying the exposure using the current aperture, shutter speed and ISO sensitivity does not exhibit sufficient performance of the exposure control system. Accordingly, as indicated by the time points t11, t12 and t13 in the graph (c) illustrated at the lower part of FIG. 4, occurrence of disorder (see the region with a symbol A in the graph (c)) in the exposure control value (for example, aperture value) provides the human eye with flickers or a sense of discomfort. The sense of discomfort or the like tends to be outstanding near the appropriate exposure value. Accordingly, as described above, the exposure is not controlled until the appropriate amount of exposure is achieved when varying the amount of exposure (see the region with the symbol A in the graph (b) illustrated at the middle part of FIG. 4).